Shielded twisted-pair cable (STP) each pair of wires is wrapped in metallic foil. The 4 pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150-Ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise, both within the cable (pair to pair coupling, or crosstalk) and from outside the cable (electromagnetic interference -- EMI -- and radio frequency interference -- RFI). Shielded twisted-pair cable shares many of the advantages and disadvantages of unshielded twisted-pair cable (UTP). STP affords greater protection from all types of external interference, but is more expensive and difficult to install than UTP.
The metallic shielding materials in STP need to be grounded at both ends. If improperly grounded (or if there are any discontinuities in the entire length of the shielding material, for example due to poor termination or installation), STP becomes susceptible to major noise problems, because they allow the shield to act like an antenna picking up unwanted signals. However, this effect works both ways. Not only does the foil (shield, screen) prevent incoming electromagnetic waves from causing noise on our data wires, but also it minimizes the outgoing radiated electromagnetic waves, which could cause noise in other devices. STP cable cannot be run as far as other networking media (coaxial cable, optical fiber) without the signal being repeated. More insulation and shielding combine to considerably increase the size, weight, and cost of the cable. And the shielding materials make terminations more difficult and susceptible to poor workmanship.
Unshielded twisted-pair cable (UTP) is a four-pair wire medium - composed of pairs of wires - used in a variety of networks. Each of the 8 individual copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires is twisted around each other. This type of cable relies solely on the cancellation effect, produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. Like STP cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable.
When used as a networking medium, UTP cable has four pairs of either 22 or 24 gauge copper wire. UTP used as a networking medium has an impedance of 100 ohms. This differentiates it from other types of twisted-pair wiring such as that used for telephone wiring. Because UTP has an external diameter of approximately .43 cm, its small size can be advantageous during installation. Since UTP can be used with most of the major networking architectures, it continues to grow in popularity.
Unshielded twisted-pair cable has many advantages. It is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling, however its real advantage is its size. Since it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when installing a network in an older building. Also, when UTP cable is installed using an RJ connector, potential sources of network noise are greatly reduced, and a good solid connection is practically guaranteed.
There are disadvantages in using twisted-pair cabling. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber optic cables. While UTP was once considered slower at transmitting data than other types of cable, this is no longer true. In fact, today, UTP is considered the fastest copper-based media.